1. Field of the Invention
The present invention relates to a semiconductor laser diode and a fabrication method thereof, and more particularly, to a semiconductor laser diode having a ridge wave guide and a fabrication method thereof.
2. Description of the Related Art
A semiconductor laser diode generates a laser beam having frequency of a narrow width and keen directivity and is thus mainly used in a field such as optical communication, a multiple communication and a space communication. Also, the semiconductor laser diode is extensively used for transmission of data or recording and reading of data in a communication field such as an optical communication and an apparatus such as a compact disc player (CDP) and a digital versatile disc player (DVDP).
The extensive use of the semiconductor laser diode is due to facts that the emission characteristics of a laser beam can be maintained in a limited space and the semiconductor laser diode is a compact device and has a small critical current value for emission. An increase in the number of industrial fields adopting the semiconductor laser diode results in an increase in a need for a semiconductor laser diode having a more reduced critical current value. That is, it is required to manufacture an excellent semiconductor laser diode capable of enabling low-current emission and having longer lifetime.
FIG. 1 is a cross-sectional view of a conventional semiconductor laser diode of a ridge wave guide structure, which is designed to reduce a critical current value for laser emission. Referring to FIG. 1, an n-GaN layer 12, which is defined by first and second regions R1 and R2, is deposited on a sapphire substrate 10. On the n-GaN layer 12 in the first region R1, an n-AlGaN/GaN layer 24, an n-GaN wave guide layer 26, an active layer 28, i.e., an InGaN layer, a p-GaN wave guide layer 30, and a p-AlGaN/GaN layer 32 are sequentially deposited. The indexes of refraction of the n-GaN wave guide layer 26 and p-GaN wave guide layer 30 are larger than those of refraction of the n-AlGaN/GaN layer 24 and the p-AlGaN/GaN layer 32 but are smaller than the index of refraction of the active layer 28. The p-AlGaN/GaN layer 32 has a ridge wave guide structure which is formed from the center of an upper portion of the p-AlGaN/GaN layer 32 that is projected. The sides of the projected center are vertical to peripheral portions and an upper portion thereof is a plane vertical to the sides.
The projected ridge wave guide structure of the p-AlGaN/GaN layer 32 confines an injected current to reduce a resonance region for laser emission in the active layer 28. A p-GaN layer 34 is deposited on the projected ridge wave guide of the p-AlGaN/GaN layer 32, and the entire surface of the p-AlGaN/GaN layer 32 is covered with a protective layer 36. Also, portions of the sides, except for a center portion, of the p-GaN layer 34 contacts the protective layer 36. A p-type electrode 38 is deposited on the protective layer 36 to contact the exposed surface of the p-GaN layer 34. The n-GaN layer 12 is more thinly formed in second region R2 than in the first region R1, and an n-type electrode 37 is deposited on the n-GaN layer 12 in the second region R2.
A conventional semiconductor laser diode has a ridge structure for confining a lot of the injected currents to reduce the width of resonance, thereby reducing a critical current value for laser emission compared to the existing semiconductor laser diode having no ridge structure.
However, in case that a portion of a sapphire substrate is thinly etched to form a ridge portion, i.e., the height of the ridge is short as shown in FIG. 2, a corner of the ridge seldom obstructs an optical profile of the laser beam, thereby reducing an optical loss. This, however, fairly increases resistance in a p-AlGaN/GaN layer 32 and a p-GaN layer 34 and makes a current injected via the p-GaN layer 34 diffused more broadly than the width of the ridge before it reaches an active layer 28. As a result, the width of a resonance region A1 is broadened, which would increase a critical current value.
Meanwhile, as shown in FIG. 3, a portion of a sapphire substrate is thickly etched, the height of the ridge is long, and a clad layer around the ridge has a thin thickness, diffusion of a current is prevented and a critical value for emission is reduced. As a result, the width of a resonance region A2 is reduced. However, an optical profile contacts the sides of the ridge, which would cause an optical loss.